A surface acoustic wave (SAW) device is a device obtained by forming comb-shaped electrodes on a piezo-electric substrate such as a lithium tantalate (LiTaO3) (LT) substrate, lithium niobate (LiNbO3) (LN) substrate and the like. This device has the function of an extremely miniature band-pass filter using electromechanical properties of piezo-electric material. In the SAW device, pitches of the order of micrometers of the comb-shaped electrodes are reflected in filter characteristics with sensitivity. Since the coefficients of thermal expansion of LT and LN are about six times that of silicon and thus large (Silicon: about 2.6×10−6/K, LT: about 16×10−6/K, LN: about 15×10−6/K), when the LT substrate or LN substrate is used in the SAW device, variations in filter characteristics with changes in temperature become a significant problem. Therefore, it has been carried out to suppress such large thermal expansion and contraction of the piezo-electric substrate or make temperature compensation by other methods.
For example, when the SAW device is manufactured, it has been carried out to suppress expansion and contraction caused by changes in temperature of a piezo-electric substrate by boding a substrate with a small coefficient of thermal expansion to the piezo-electric substrate. It is disclosed in Patent Document 1 that an LT substrate and sapphire substrate are bonded using a direct joint method. Further, it is disclosed in Patent Document 2 that a piezo-electric substrate and monocrystal substrate are joined using junction by solid phase reaction. Furthermore, it is disclosed in Patent Document 3 that an LT (LN) substrate and silicon substrate are joined using junction by hydrophilic processing and heat treatment.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-343359    Patent Document 2: Japanese Laid-Open Patent Publication No. H09-208399    Patent Document 3: Japanese Patent Gazette No. 2607199
In recent years, in cellular telephones and the like installed with the SAW device, many systems coexist with one another, and it is expected that frequency bands used in the systems are adjacent to one another. In such a case, it is required to minimize a frequency shift (the order of several megahertz). Accordingly, the piezo-electric substrate is required to vary filter characteristics due to changes in temperature as little as possible. However, in the piezo-electric substrate obtained by the method of bonding a substrate with a small coefficient of thermal expansion as in the conventional technique, it is not possible to deal with the requirement of making the frequency shift smaller. Accordingly, under present circumstances, the piezo-electric substrate capable of dealing with the requirement of making the frequency shift smaller is not yet in existence.